The invention relates to an apparatus for collecting water or other condensate from the interior of a nuclear reactor containment structure, and recycling the condensate for reuse in an emergency heat exchange process whereby excess heat is removed from the core and transferred to the containment structure.
Modern nuclear reactors such as pressurized water reactors have a pressurized reactor vessel containing the nuclear fuel, coupled to a primary reactor circuit wherein the primary coolant is circulated over the fuel in the reactor vessel and through an electrical generator which extracts the energy imparted to the primary coolant by the fuel. The reactor is surrounded by a containment structure intended to prevent a large scale leak of radioactivity into the environment in the event of a reactor mishap. For example, if the primary coolant system provided for transferring heat from the reactor core to the electrical generator fails, e.g., from loss of primary coolant pressure due to a leak or rupture in a connecting line, the reactor may quickly overheat. Some known reactors include a large water storage tank inside the containment structure, which provides a thermal mass for use as a heat sink in the event of a nuclear accident. A heat exchanger located in the water storage tank transfers heat from the reactor vessel and fuel to the water in the storage tank, thereby providing both a sink and a thermal transfer means to carry heat away from the core.
U.S. Pat. No. 4,668,467--Miler et al. discloses a safety cooling installation for a nuclear reactor wherein a large reservoir stores cooling water outside of the containment structure. In the event of a nuclear emergency, cooling water is directed from the reservoir into the containment structure, being emitted through a spray manifold near a top of the containment structure, or simply pumped into the containment structure to accumulate at a sump in the bottom of the containment structure, which is allowed to fill until the cooling liquid reaches a predetermined level. If the emergency is due to a rupture of the primary reactor circuit, the cooling water becomes mixed with the primary coolant. When the cooling liquid has, reached the predetermined level, the liquid is pumped from the sump through an external air heat exchanger, and back into the containment structure, in a heat exchange loop intended to carry away the heat of the core. If necessary, additional water can be injected from the reservoir.
The cooling spray acts to moderate the temperature of the nuclear reactor and thus to reduce pressure in the containment structure. However, Miler requires a water supply that is external of the containment structure and a number of elements such as a pump and heat exchanger, which require regular maintenance.
U.S. Pat. No. 3,929,567--Schabert et al teaches a safety system which is operable to flood the core in the event of an accident. A quantity of water is stored in tanks high in the containment structure, and released in the event of an accident. The high position of stored water provides a pressure head to help overcome any pressure in the primary circuit. Once released, a heat exchange loop including a heat exchanger located outside of the containment structure operates to remove excess heat. Like Miler, this arrangement relies on pumps and valves for moving the cooling water through a heat exchange loop, requiring maintenance as well as expenditure for conduits, valves and pumps.
The present invention provides a thermal heat sink in the form of a quantity of cooling water which can flood the containment structure or be coupled thermally to the reactor core via heat exchange means. However, the invention provides a safety cooling system which collects and recirculates cooling water in a passive operation.
In the event of a nuclear accident, the water in even a large heat sink tank may heat to boiling in several hours. The water remains at a constant boiling temperature (assuming constant pressure) until the heat energy is sufficient to change the phase of all the water from liquid to gas, i.e., steam. Steam produced by boiling the water in the heat sink tank is released into the interior of the containment structure. Whereas the walls of the containment shell are relatively cooler than the steam, the water condenses on the walls and drains down into lower parts of the containment structure where it would remain. Without means for recovering this water, the contents of the heat sink tank could completely boil away into the containment, for example over several days. If another cooling means has not been secured before, the reactor temperature will rise uncontrollably and a melt down of the reactor core may occur.
In order to reduce the dependence on the operator and to prevent such a melt down, there is a need to permit the heat exchanger to operate for an essentially indefinite time. This could be accomplished by replenishing the water in a heat sink water storage tank. .The water could be replenished from an outside source, but the problem still remains keeping the storage tank supplied with water for an essentially indefinite time. Further, for reliability reasons it would be desirable to have a passive supply of water that is not dependent upon an outside source, or upon pumps, valves, and other devices which require maintenance.
The present invention provides an apparatus for passive recirculation of a cooling liquid inside of a nuclear containment structure. Vaporized cooling liquid condenses on the interior of the containment structure which is itself cooled by a passive system utilizing natural circulation of air and evaporation of water U.S. Pat. No. 4,753,771--Conway). The condensate is collected in suitable drip collection gutters and conduits, and returned to the heat sink water storage tank by gravitational force. The cooling liquid can be recirculated indefinitely, and although it is possible to supplement cooling by externally cooling the containment structure, no external supply of cooling liquid is required. Moreover, radioactivity released in the containment structure remains there because there is no requirement for a flowpath including an external heat exchanger.